I'm going to take a swing at this and hopefully I can help. The difference between the tech trees is simple. You have 4 categories, in each they give you a basic few techs. If your looking to blow something up then unlocking military type techs is likely going to get you what your looking for. You can expand into the tree view to look around for something specific. The only other way I could read that question was if you meant between different races. If that is the case its worth noting that you can only use one at a time, but you can get a synopsis from reading about the various races strengths and weaknesses. Another more direct synopsis can be gleamed by making a custom race and flipping through the tech trees. Beyond that you could explore the xml files  if you for some reason really needed to see every single tech in each tree. There is an in progress wiki. Give it a chance to grow. You can take a look at previous stardock wikis and you will know your concerns will be addressed with some time. 

1st Max difficulty on this game is probably a REALLY bad idea for you until you have alot more gameplay under your belt. No wait let me rephrase that. Max difficulty is a REALLY bad idea for ANYONE who wants any sort of chance of winning. Godlike is... godlike. Don't say you weren't warned. I suppose if your looking for that "What is the perfect combo to be the best" type info I would have to say you won't find it at all. The reason for that is probably different then you would expect. Simply put, that doesnt exists anywhere because it just depends on the game your in. It depends upon what you have, where you are, what AI's are playing and what strategy those AI's are pursuing. This isn't like a game where you can encounter X race and say ok I know the way to beat them is going to be to do X. They adapt and change up their goals and strategies, based upon what they encounter as well. That being said it will almost always be easier to get along with the Terrans then the Drengins... But maybe not. Long story short. "optimal" depends upon the very specific game your in.

What do modifiers mean depends upon which it is. Mousing over production on planets will usually answer that question. The in game explanations are almost always a work in progress but the information is by and large available at this point in some fashion.

Hopefully this was helpful, and I would be happy to answer any other questions I can.

The Wiki is designed to be the place to go for this sort of information some of the data there is direct from stardock, but this is a huge game with lots to know. 

If you go and look at the standard race descriptions or go to make a custom race and scroll through the tech trees you get an idea of what each tech tree is focused on give to you, it doesn't do a tech by tech comparison though as that would be a huge under taking and far from light reading.

Anyway while I hope to see a lot more documentation through the Wiki, there is actually a lot of in game information in the tool tips.

What the bonus tiles do is increase the level of the improvements that are built on or next to the hex with the bonus.  When you build a manufacturing building on top of a hex that give +3 manufacturing, the actual bonus you get is dependent on the building itself, not the tile.  Most will get +5% manufacturing, while some will get +5% production.  Population buildings may get +0.5 food or +5% population growth.  That's why the information is simple, like "+3".  Adjacency gives +1 level to each similar building next to it.  Bonuses give +X level.  If a building has at least one bonus level (either through adjacency or hex bonuses) certain technologies will increase that level, but only once.

So it is a little bit complicated.

As for the OP:

Including documentation for everything you want would be a waste of time and resources for the developer.  The vast majority of players are going to learn the game by playing it.  For those who like to read and dig into the information (which is absolutely fine, you're just in a very small minority) there's the wiki but it is currently very new!  You're going to have to wait for the pages to fill up with information, sorry.  I would strongly warn against playing max map, max difficulty in this game.  The biggest map is ridiculously big.  "Insane" is a very, very good descriptor.  It is for people who want to spend weeks or months on a single game.  I wouldn't recommend getting that invested into a single game until you've learned the ins and outs of the game by experiencing it. (not just reading about it)  The Large-Gigantic size maps should be big enough for anyone who likes big maps. (like me!)  These names for the map sizes should be taken at face value.  Large is large!  Huge is huge!  Excessive really is excessive.

The advice I'm giving you is advice I'm giving to myself: tone down those big-map ambitions.  This scope of this game is massive.  You (and I) should spend some time on smaller maps learning what we like, what we don't like, and getting familiar with various tech trees and stuff.  You can look at the trees, you can read about them, but often it is only in practice that you can really get a feel for them. And I'm telling myself this with almost 200 hours in the game - I need to spend more time on smaller maps to learn the game better!

Quick question in a similar vein for anyone helpful with a moment to spare

If an enemy civ has rushed a starbase to minable resources next to my starting area, how can I get my own starbases to assume control of these resources? Must I destroy their starbase as it was there first (aka war)? Or is there a way to do it through border expansion? Currently I have them completely engulfed and even the resources themselves are now back in my own borders, but their base is still controlling them.

First, thanks Mohobie, Lavo_2, Turkwise and Larsenex for the advice given

Yes, creating a custom race is what I intended. The problem for me is, there are different tech trees to choose from (like Terran, Basic, Altarian...) and every tech tree has some (seemingly) special techs and with them unique buidling with of course unique stats. I can't remember them all, so I either have to write them all down and make some excel list to just compare the differences. Then I would be able to weigh the different advantages and disadvantages etc. and play the optimization-game. 

You are right of course, I obviously didn't make myself clear, but should have forseen this misunderstanding. An optimal strategy is both discrete and flexible, will suit for every situation, is both simple and infinitly complex. A strategy then becomes an infinte well of imperatives for every scenario. This is what I mean by "optimal". In this definition it well exists, although it is less like a book and more like a library or a rubic cube.

I see your point of course, but I'm pretty sure that the developers themself have excel sheets with tech trees for example. I guess it would be a days worth of work, maybe two, to convert that to a wiki-readable format. The issue here for me is, weigh 2 days work for a developer against (X customers times Y hours of confusion) or the time the fans need to write the wiki, which I estimate to be much higher than the time the devs would need because they don't have to reverse engineer or gather the information in a collaborative effort. After having written a large part of the Europa Universalis 4 Wiki I gotta say my motivation for writing another wiki just to play the game in any orderly fashion - in my view of course - is quite low.

Also, the wiki will eventually get built up. For a minority this will be part of the experience, for the majority it will be frustrating or at least unhelpful to don't find the right info in the wiki, especially shortly after launch. So I guess I will either have to write the wiki or sit back for a few weeks to wait for other people to have written the tech tree-articles in the wiki because I don't get paid to invest that much time - unlike the developers.

So in conclusion, I'd like to point out that I understand the difficulties the developers face, but providing documentation for such a game is part of the product. The shift of work for the creation of the documentation from the developers to the customers is not only inefficient, ineffective, not customer-orientated but above all getting on my nerves

PS: Anyone has a comparison sheet for all the techs and buildings in GC3?

Ahh, super that's good to know, much obliged!

It also serves as, intentionally or not, a nerf of the Colonizers trait, as you will always only be able to build a basic improvement, and not an upgraded version.